It is well known to provide an electronic switch to replace a mechanical switch which may be manually operable or may be constituted by contacts-of a relay. It is also well known that, in order to provide desired characteristics of a low on-resistance (switch closed) and a high off-resistance (switch open), FETs (field effect transistors) are desirably used as the electronic or solid-state switching elements. For example, "Modern Electronic Circuits Reference Manual" by John Marcus, McGraw-Hill Book Company, shows on page 957 under the heading "DPDT FET" an arrangement of four commonlycontrolled FETs constituting a double-pole double-throw switch, and shows on page 959 under the heading "FET Analog Switch" an arrangement of a FET switch with a level-shifting bipolar transistor driver.
The latter and other arrangements are disclosed in an article entitled "Solid-state signal switching: it's getting better all the time" by Joel Cohen, EDN, Nov. 15, 1972, pages 22-28. FIG. 12 of this article shows an analog switch in which an n-channel MOSFET (metal-oxide-semiconductor FET) is used as the switching element with a bipolar transistor driver connected directly to the gate of the MOSFET for applying forward or reverse bias for controlling the switch.
A problem with these arrangements is that, if the source of the MOSFET is connected to the voltage to be switched and the drain of the MOSFET is connected to a load, then the difference between the voltage to be switched by the switch and the control voltage at the gate of the MOSFET is limited by the maximum source-gate voltage permitted for the FET. For most MOSFETs, this maximum voltage difference does not exceed about 20 volts, and this considerably limits the applications of these known electronic switching arrangements. For example, if such a known electronic switch is to be controlled by low voltage logic signals, for example 0 and +5 volts from a CMOS logic circuit, then the voltage to be switched can not exceed about 20 to 25 volts. Such a switch can not be used to switch relatively high voltage signals, such as signals in telecommunications equipment which may have voltages in a range of about -48 to -200 volts.
The known electronic switching arrangements described above are intended to be used with the MOSFET having its gate supplied with control voltages of respectively +15 and -15 volts received from a grounded voltage source, its drain connected to a lesser voltage to be switched, for example within a range of .+-.10 volts relative to ground, and its source connected via the load to a fixed potential such as ground or 0 volts. Connected in this manner as intended, such known arrangements are not suitable for switched connection of the load between relatively arbitrary potentials in a so-called floating manner, as may also be desirable for switching signals in telecommunications equipment, and are also not suitable for switching relatively high voltage signals.
These limitations could be reduced by controlling the gate of the MOSFET in a manner isolated from the ground potential via an intermediate d.c. to d.c. converter, but this has consequent disadvantages of extra cost, power dissipation, and physical space.
An object of this invention is to provide an improved electronic switch.